Process for Producing a Gas Stream Depleted of Hydrogen Sulphide

ABSTRACT

The invention provides a process for producing a gas stream depleted of H 2 S from a feed gas stream comprising H 2 S, the process comprising the steps of: (a) selectively oxidizing H 2 S by supplying the feed gas stream comprising H 2 S, an inert liquid medium and a molecular-oxygen comprising gas stream to a reaction zone comprising at least one catalytic zone comprising an oxidation catalyst and contacting the oxidation catalyst of each catalytic zone with the feed gas stream and/or the molecular-oxygen comprising gas stream in the presence of inert liquid medium at a temperature in the range of from 120 to 160° C. to form elemental sulphur and the gas stream depleted of H 2 S, under such conditions that the elemental sulphur formed is essentially in liquid form and is removed from the reaction zone with inert liquid medium to obtain a liquid stream comprising inert liquid medium and essentially liquid elemental sulphur; (b) optionally separating the liquid stream obtained in step (a) into a first liquid phase enriched in inert liquid medium and a second liquid phase enriched in liquid elemental sulphur; (c) combusting at least part of the liquid stream obtained in step (a) or at least part of the second liquid phase obtained in step (b) to form a fluid stream comprising sulphur dioxide.

The invention relates to a process for producing a gas stream depletedof hydrogen sulphide (H₂S).

A process known in the art for producing a gas stream depleted of H₂Sfrom a gas stream comprising H₂S is the oxidation of H₂S to SO₂according to:

2H₂S+3O₂→2H₂O+2SO₂  (1)

The SO₂ formed can be (catalytically) converted to elemental sulphuraccording to the Claus reaction:

2H₂S+SO₂→2H₂O+3/nS_(n)  (2)

The combination of reactions (1) and (2) is known as the Claus process.The Claus process is frequently employed both in refineries and for theprocessing of H₂S recovered from natural gas. A disadvantage of theClaus process is that it is thermodynamically limited by the increase ofthe water vapour content and simultaneously by the decrease of the H₂Sand SO₂ concentration as the H₂S conversion proceeds, with the resultthat the equilibrium of the Claus reaction shifts to the left. Since thedew point of water lies below the solidification point of sulphur,condensation of the water vapour in the process gas to remove thislimitation can lead to serious problems, such as clogging due to thesolidification of sulphur and corrosion due to the formation ofsulphuric acid, causing serious operational problems. Anotherdisadvantage of the Claus process is that the concentration of H₂S inthe treated gas is still relatively high. The Claus process has beenimproved to an extent where the H₂S content of the treated gas can belowered to approximately 1 vol % (Superclaus-99 process). However,especially in the case where it is necessary to comply with requirementswith regard to residential use or environmental regulations with respectto emission of sulphur compounds, even lower concentrations of H₂S, inthe ppm range, have to be achieved.

A disadvantage of the Claus and Superclaus process is that such lowconcentrations of H₂S cannot be achieved.

A further disadvantage is that H₂S needs to be separated from the gasstream comprising H₂S (which can for example be a natural gas stream)and the resulting stream enriched in H₂S is then fed to a Claus unit.The reason for this is that the oxidation reaction according to reaction(1) will also cause oxidation of species other than H₂S. Additionally,the Claus process needs a feed enriched in H₂S to proceed at anacceptable rate. One generally applied method to achieve separation ofH₂S is to contact the gas stream comprising H₂S with an absorptionsolvent, typically one or more amine solvents, to form a solvent loadedwith H₂S. The regeneration of this loaded solvent stream results in agas stream enriched in H₂S, which can then subsequently be treatedaccording to reactions (1) and (2) in the absence of hydrocarbons orhydrogen. Thus, a conventional process for producing a gas streamdepleted of H₂S comprises the steps of absorption of H₂S in an amineabsorption unit, followed by regeneration of the amine liquid to producea gas stream loaded with H₂S and finally conversion of H₂S to sulphur ina Claus unit. This results in a cumbersome process and large amounts ofamine solvents are needed.

Another known process for producing a gas stream depleted of H₂S from agas stream comprising H₂S is the selective oxidation of H₂S to elementalsulphur, described for example in U.S. Pat. No. 4,886,649. The selectiveoxidation in U.S. Pat. No. 4,886,649 is performed in two stages. Thefeed oxidation stage is carried out in a fluidised bed of a granularcatalyst containing 10-20% by mass of magnesium chromate on aluminiumoxide at temperatures between 250-350° C. In the second oxidation stage,unreacted H₂S and oxygen from the feed stage are reacted at 140-155° C.in the presence of a catalyst containing vanadium pentoxide andaluminium oxide.

A disadvantage of the process of U.S. Pat. No. 4,886,649 is that gaseoussulphur is formed in the feed stage. At the concentrations whereinsulphur is present in the gaseous effluent of the feed stage, this willinevitably result in the formation of a sulphur mist, which is difficultto separate from the gas flow and results in deposition of elementalsulphur on the catalyst, reactor elements or conduits.

In U.S. Pat. No. 4,311,683 a process for producing a gas stream depletedof H₂S is described wherein H₂S is removed from a feed gas stream byselective oxidation of H₂S. The feed gas stream comprising H₂S andoxygen is passed through a catalyst bed under conditions such that H₂Sand oxygen react to produce elemental sulphur vapour. An inlettemperature into the catalyst bed of between about 250° and 450° F.(121° and 232° C.) is mentioned. An oxidation catalyst comprising anoxide and/or sulphide of vanadium supported on a non-alkaline porousrefractory oxide is used. It is mentioned that sulphur deposition andconsequent catalyst deactivation are prevented by maintaining thepartial pressure of free sulphur in the oxidation reactor below thatnecessary for condensation. Preferably, the temperature is maintainedbelow 450° F. (232° C.) and the H₂S concentration in the feed is keptlow by diluting the feed with an inert gas or with recycle gases.

In U.S. Pat. No. 6,207,127 a selective oxidation process in anadiabatically operating reactor is described wherein the inlettemperature of the catalyst bed is at least 150° C., preferably at least170° C., i.e. above the dew point of the sulphur formed. The catalystcomprises a mixed oxide of iron and zinc on a silica support.

In the processes described in U.S. Pat. No. 4,311,683 and U.S. Pat. No.6,207,127, sulphur is kept in the vapour phase by performing theselective oxidation at temperatures above about 160° C. and by keepingthe sulphur concentration very low. This means that these processes arenot suitable for deep desulphurisation of gases having a high content ofH₂S, since this would inevitably lead to sulphur deposition.

A further disadvantage of all the processes mentioned hereinbefore isthat a considerable amount of sulphur is produced as waste.

Yet another disadvantage of all the processes mentioned hereinbefore isthat they do not enable the removal of mercaptans, in the event that itis desired to produce a gas stream depleted of H₂S as well as ofmercaptans from a feed gas stream comprising mercaptans as well as H₂S.The removal of mercaptans from a gas stream is generally more difficultcompared to the removal of H₂S. The removal of mercaptans is ofimportance in view of increasingly stringent environmental and technicalrequirements. Mercaptans, due to their odorous nature, can be detectedat parts per million concentration levels. Thus, it is desirable incases where the gas stream is intended for domestic use, to haveconcentrations of mercaptans lowered to e.g. less than 5, or even lessthan 2 ppmv.

Mercaptan removal is also important in cases where the gas stream is acarrier gas stream, for example an inert gas or a hydrocarbonaceous gasthat has been used to strip a mercaptan comprising reactor bed and isloaded with mercaptans. The removal of mercaptans from such a loaded gasstream is necessary to be able to use the gas stream again as strippinggas.

Processes for the removal of mercaptans from a gas stream are generallybased on physical absorption, solid bed adsorption and/or chemicalreaction.

Physical absorption processes generally suffer from the fact that largereactors are needed to achieve the desired low concentrations ofmercaptans.

Solid bed adsorption processes suffer from the fact that they are onlyable to adsorb limited amounts of undesired compounds, whileregeneration is relatively cumbersome, see for example U.S. Pat. No.4,311,680.

Chemical processes suffer from the fact that they do not effectivelyremove mercaptans, often have a high consumption of chemicals and oftenproduce large amounts of waste, see for example EP 229,587.

In view of the difficulties encountered in the known processes, there isa need in the art for a process for producing a gas stream depleted ofH₂S from a gas stream comprising H₂S in a relatively high amount withoutthe operational problems of known processes and without the productionof a considerable amount of waste sulphur. This is important because thesulphur market is not able to absorb increasing supplies of elementalsulphur, as can be observed by increasing storage of elemental sulphur(also denoted as “yellow mountains”). Further, it is desired to have aprocess for producing a gas stream depleted in H₂S as well as mercaptansfrom a feed gas stream comprising H₂S as well as mercaptans.

It has now been found that the above can be achieved by selectivelycatalytically oxidising H₂S to elemental sulphur in the presence of aninert liquid medium such that the sulphur formed is essentially inliquid form and can be removed from the catalyst with the inert liquidmedium, and combusting the elemental sulphur thus formed to SO₂.

Accordingly, the invention provides a process for producing a gas streamdepleted of H₂S from a feed gas stream comprising H₂S, the processcomprising the steps of:

(a) selectively oxidizing H₂S by supplying the feed gas streamcomprising H₂S, an inert liquid medium and a molecular-oxygen comprisinggas stream to a reaction zone comprising at least one catalytic zonecomprising an oxidation catalyst and contacting the oxidation catalystof each catalytic zone with the feed gas stream and/or themolecular-oxygen comprising gas stream in the presence of inert liquidmedium at a temperature in the range of from 120 to 160° C. to formelemental sulphur and the gas stream depleted of H₂S, under suchconditions that the elemental sulphur formed is essentially in liquidform and is removed from the reaction zone with inert liquid medium toobtain a liquid stream comprising inert liquid medium and essentiallyliquid elemental sulphur;(b) optionally separating the liquid stream obtained in step (a) into afirst liquid phase enriched in inert liquid medium and a second liquidphase enriched in liquid elemental sulphur;(c) combusting at least part of the liquid stream obtained in step (a)or at least part of the second liquid phase obtained in step (b) to forma fluid stream comprising sulphur dioxide.

The process according to the invention offers a number of advantages.

A significant advantage is that the process according to the inventionenables the production of a gas stream depleted of H₂S in two steps,without producing substantial sulphur waste. The process according tothe invention is considerably simplified in view of a conventionalprocess wherein amine absorption is needed, followed by a Claus process.In the process according to the invention, no amine solvent is needed.

One advantage is that unwanted build-up of sulphur in the catalytic zoneis prevented. By performing the catalytic selective oxidation in thepresence of an inert liquid medium at a temperature in the range of from120 to 160° C., the sulphur formed is essentially in liquid form and canbe removed from the catalyst with the inert liquid medium as a liquidsulphur phase. This facilitates the conversion of H₂S because no sulphurdeposition on the catalyst surface will take place. This enables theproduction of a gas stream depleted of H₂S having an H₂S concentrationin the ppmv range (a H₂S conversion higher than 99.9% is possible).

Another advantage is that the oxidation of H₂S to elemental sulphur isselective in the sense that compounds other than H₂S, such ashydrocarbons or hydrogen, are not or hardly oxidized.

Yet another advantage is that elemental sulphur is converted intosulphur dioxide, thereby considerably reducing the amount of liquid orsolid waste.

A further advantage is that in the event that mercaptans are present inthe feed gas stream, they will be removed from the catalytic zone aspart of the liquid stream comprising inert liquid medium and essentiallyliquid elemental sulphur and will eventually be combusted to sulphurdioxide, carbon dioxide and water in step (c). Hence, the processaccording to the invention offers a relatively simple way to produce agas stream depleted of H₂S as well as mercaptans.

Finally, the heat released in the combustion step (c) can be usedadvantageously, for example to generate power and/or high-pressure steamto be used elsewhere.

DETAILED DESCRIPTION OF THE INVENTION

In step (a) of the process according to the invention, a feed gas streamcomprising H₂S, a molecular oxygen-containing gas stream and inertliquid medium are supplied to a reaction zone comprising at least onecatalytic zone comprising an oxidation catalyst.

The oxidation catalyst may be any oxidation catalyst suitable for theselective oxidation of H₂S. Suitable catalysts for the selectiveoxidation of H₂S are generally in the form of a refractory oxidematerial on which a catalytically active material has been deposited.The oxidation catalyst may comprise as catalytically active material anymaterial that is capable of performing an oxidation reaction. Suchoxidation catalysts are known in the art and typically comprise an oxideand/or a sulphide compound of one or more metals. Reference herein to anoxide compound of one or more metals is to a compound of the approximategeneral formula MS_(x-1)O_(y), wherein M is one or more metals, and xand y have, independently, a numeric value of at least 1. Referenceherein to a sulphide compound of one or more metals is to a compound ofthe approximate general formula MS_(x)O_(y-1). When contacted with H₂S,the metal oxide compound will be converted to a metal sulphide compoundand water is formed. When the thus-formed metal sulphide compound isthen contacted with oxygen, it is converted into the metal oxidecompound and elemental sulphur is formed. These two subsequent reactionsare symbolically represented by the following equations:

MS_(x-1)O_(y)+H₂S→MS_(x)O_(y-1)+H₂O  (3a)

MS_(x)O_(y-1)+½O₂→MS_(x-1)O_(y)+S  (3b)

The overall reaction is the selective oxidation reaction according toequation (3).

2H₂S+O₂→2H₂O+2/nS_(n)  (3)

It will be appreciated that the proportion of oxygen and sulphur in thecatalyst metal compound will vary during the catalytic process. Thecompound having the highest proportion of oxygen is represented asMS_(x)O_(y-1) in equations (3a) and (3b) and referred to as oxide. Thecompound having the highest proportion of sulphur is represented asMS_(x-1)O_(y) and referred to as sulphide.

The metal M may for example be vanadium, chromium, manganese, iron,cobalt, molybdenum or combinations thereof. Examples of prior artcatalysts for the selective oxidation of H₂S are iron oxide-chromiumoxide on silica, iron oxide-phosphorus oxide on silica, ironoxide-sodium oxide on silica (EP-A-0409353) magnesium chromite onalumina, vanadium pentoxide on alumina (U.S. Pat. No. 4,886,649) andsilicon carbide supporting an active phase comprising nickel in theoxysulfide form (U.S. Pat. No. 6,235,259). Preferably, the catalyticallyactive material is an oxide and/or sulphide compound of iron or an ironcomprising mixed metal oxide and/or sulphide compound, more preferablythe catalytically active material comprises a hydrated iron oxidecompound.

The reaction zone in step (a) comprises at least one catalytic zone. Insome embodiments of step (a) of the process according to the invention,both reactions (3a) and (3b) take place in each catalytic zone. In theseembodiments, the feed gas stream comprising H₂S and the molecular oxygengas stream are both supplied to each catalytic zone. In each catalyticzone, the catalytically active compounds of the oxidation catalyst, i.e.the oxide or sulphide compounds of a metal, will alternately be in itsoxide (MS_(x-1)O_(y)) and sulphide (MS_(x)O_(y-1)) form.

In alternative embodiments of step (a), reaction (3a) takes place in onecatalytic zone and reaction (3b) takes place in a different catalyticzone. The feed gas stream comprising H₂S is then supplied to thecatalytic zone where reaction (3a) takes place and the molecular-oxygencontaining gas stream is then supplied to the catalytic zone wherereaction (3b) takes place. During the process, the oxidation catalyst inthe catalytic zone wherein reaction (3a) takes place will be convertedfrom its oxide form (MS_(x-1)O_(y)) into its sulphide form(MS_(x)O_(y-1)) and the oxidation catalyst in the catalytic zone whereinreaction (3b) takes place will be converted from its sulphide form intoits oxide form. It will be appreciated that in these alternativeembodiments, the oxidation catalyst can be considered as a regenerableadsorbent.

In all embodiments of step (a), the inert liquid medium is preferablyrecycled at least partly to the catalytic zone. In case that the inertliquid medium is not liquid sulphur, at least part of the sulphur ispreferably removed from the inert liquid medium before recycling it. Inthat case, the greater part of the sulphur may be separated from theliquid stream by phase separation.

In a first embodiment of step (a) of the process according to theinvention, the reaction zone comprises a single catalytic zone ofoxidation catalyst and the feed gas stream comprising H₂S, themolecular-oxygen comprising gas and the liquid inert medium are suppliedto that single catalytic zone. In this first embodiment of step (a), thefeed gas stream comprising H₂S and the stream comprising oxygen arecontacted with the oxidation catalyst in the presence of inert liquidmedium. The temperature of the single catalytic zone is maintained inthe range of from 120 to 160° C. The heat released by the exothermicoxidation reaction is at least partly absorbed by the inert liquidmedium. Due to the heat absorption by the inert liquid medium and,optionally, by additional internal or external cooling means, thetemperature in the single catalytic zone is kept below the temperatureat which a significant viscosity increase due to sulphur polymerizationtakes place, i.e. below about 160° C.

A gas-liquid mixture comprising a gas stream depleted in H₂S and inertliquid medium with the sulphur formed dissolved in it, mixed with it orfinely dispersed or emulsified in it, is removed from the catalyticzone. The gas and liquid are separated into a gas stream depleted in H₂Sand a liquid stream comprising inert liquid medium and essentiallyliquid elemental sulphur. The liquid stream may comprise more than oneliquid phase, for example a phase of inert liquid and a separate phaseof liquid sulphur and/or water.

In a second embodiment of step (a), the reaction zone of the processaccording to the invention comprises two or more catalytic zones ofoxidation catalyst in series. Both reactions (3a) and (3b) then takeplace in each catalytic zone. The feed gas stream comprising H₂S and theoxygen gas stream are supplied to and contacted with the oxidationcatalyst of each catalytic zone.

The use of several catalytic zones in series is advantageous in the caseof a feed gas stream having a high content of H₂S, typically up to 50vol %, based on the total feed gas stream. In that case, severalcatalytic zones in series can provide for the possibilities ofinterstage cooling, interstage water separation, staged supply of feedgas or of molecular-oxygen containing gas or a combination of two ormore thereof.

In the case of several catalytic zones in series, at least part of thefeed gas stream comprising H₂S, at least part of the oxygen comprisinggas stream and at least part of the inert liquid medium are supplied tothe first, i.e. the most upstream, catalytic zone, which is operated ashereinbefore described for the first embodiment.

Preferably, the effluent of the first catalytic zone, i.e. a gas-liquidmixture comprising a gas stream depleted of H₂S and inert liquid mediumwith the sulphur formed dissolved in it, mixed with it or finelydispersed or emulsified in it, is sent to the second catalytic zone,optionally after cooling. The remainder of the feed gas stream and/ormolecular-oxygen containing gas stream is then supplied to the secondcatalytic zone. It will be appreciated that if there are more than twocatalytic zones, the remainder of the feed gas and/or molecular-oxygencontaining gas may be divided over the second and further downstreamcatalytic zones. The effluent of the catalytic zone which is locatedmost downstream will be separated into a H₂S depleted gas stream and aliquid phase comprising inert liquid medium and sulphur. Preferably,inert liquid medium is recycled back to the first catalytic zone,typically after removal of at least part of the elemental sulphur.

It is possible to separate the effluent from each catalytic zone intogas and liquid and to recycle inert liquid medium to that catalyticzone. In that case, new inert liquid medium has to be supplied to thenext downstream catalytic zone.

The process according to the present invention is very suitable forpurifying a feed gas stream having a relatively high content of H₂S,i.e. up to 50 vol %, based on the total feed gas stream. Preferably, theH₂S concentration in the feed gas stream comprising H₂S is in the rangeof from 0.5 to 50 vol %, more preferably of from 1 to 25 vol %, based onthe total feed gas stream.

In the case of a relatively high H₂S concentration of the feed gas,typically in the range of from 5 to 25 vol %, it is advantageous toapply inter-stage water separation by separating an inter-stage effluentinto its gaseous and liquid part and condense water from the gaseouspart before it is supplied to the next downstream catalytic zone.Inter-stage water separation is preferably applied in combination withstaged supply of the molecular-oxygen comprising gas stream and/or feedgas stream. In the case of a very high H₂S concentration sulphide of thefeed gas, typically in the range of from 10 to 50 vol %, based on thetotal feed gas stream, it is advantageous to perform the process in suchway that reactions (3a) and (3b) are carried out in separate catalyticzones. This enables the use of air as molecular-oxygen comprising gas,without causing the H₂S depleted gas stream to be diluted with nitrogengas from the air.

If the reactions (3a) and (3b) are carried out in separate catalyticzones, the reaction zone comprises catalytic zone A and catalytic zoneB, both zones comprising an oxidation catalyst comprising an oxideand/or sulphide compound of one or more metals. The oxidation catalystin zone A comprises the oxide compound and the oxidation catalyst ofzone B comprises the sulphide compound of the metal(s). The feed gasstream comprising H₂S and the inert liquid medium are supplied tocatalytic zone A and contacted with the oxidation catalyst of zone A toconvert the metal oxide compound into its sulphide and to form a gasstream depleted in H₂S. Air and inert liquid medium are supplied tocatalytic zone B and contacted with the oxidation catalyst of zone B toconvert the metal sulphide compound into its oxide and to form elementalsulphur.

A gas stream depleted in H₂S and a liquid stream comprising inert liquidmedium are separately recovered from zone A. Inert liquid mediumrecovered from zone A is preferably recycled to zone A or supplied tozone B or both. From zone B, a gaseous stream of depleted air and aliquid stream comprising inert liquid medium and essentially liquidelemental sulphur are separately recovered. The inert liquid mediumrecovered from zone B is preferably recycled to zone B or supplied tozone A, typically after removal of at least part of the sulphur.

Preferably, the oxidation catalyst used in zone A is, when the metaloxide compound is for a substantial part converted into its sulphide,used as the oxidation catalyst in zone B and vice versa, i.e. theoxidation catalyst used in zone B is, when the metal sulphide compoundis for a substantial part converted into its oxide, used as theoxidation catalyst in zone A. In the case that the catalytic zones eachcomprise a fixed bed of oxidation catalyst, this is typically done in aso-called swing mode operation. In the case that the catalytic zoneseach are in the form of a slurry-bubble column comprising a slurry ofparticles of the oxidation catalyst in inert liquid medium, this may bedone by removing slurry from zone B and, optionally after removal ofsulphur, supplying it to zone A and vice versa. The slurry removed fromzone B comprises catalyst particles comprising a metal oxide compound,inert liquid medium, and liquid sulphur. In order to prevent build-up ofsulphur, preferably at least part of the sulphur is removed from theslurry before it is supplied to zone A. The slurry removed from zone Acomprises catalyst particles comprising a metal sulphide compound andinert liquid medium.

The feed gas stream comprising H₂S is preferably supplied to one or moreof the catalytic zones in the reaction zone at a gas hourly velocity inthe range of from 100 to 10,000 Nl/kg/h (normal litres of gas perkilogram of catalyst in that zone per hour), more preferably of from 200to 5,000 Nl/kg/h. Reference herein to normal litres is to litres of gasat conditions of Standard Temperature and Pressure, i.e. 0° C. and 1atmosphere.

The amount of inert liquid medium supplied to a catalytic zone ispreferably such that the ratio of gas-to-liquid supplied to that zone isin the range of from 10 to 10,000 Nl gas/kg liquid, more preferably offrom 20 to 2,000 Nl gas/kg liquid. It will be appreciated that the exactgas-to-liquid ratio mainly depends on the amount of H₂S that is to beoxidized in that catalytic zone, since the inert liquid serves to absorbthe reaction heat in order to keep the reaction temperature of that zonebelow the temperature at which a significant viscosity increase due tosulphur polymerisation takes place, i.e. below 160° C.

In those embodiments wherein reactions (3a) and (3b) take place inseparate catalytic zones, the catalytic zone wherein reaction (3a) takesplace, i.e. catalytic zone A, is also supplied with inert liquid mediumin order to absorb the heat of exothermic reaction (3a). In thepreferred embodiment wherein the inert liquid medium is sulphur, theinert liquid medium has in zone A the additional function of keeping thesulphur in the liquid form and to remove it from zone A.

If the feed gas stream has a very high H₂S content, typically above 10%,it might be preferred to apply additional cooling of the reaction zone,i.e. additional to the cooling effected by the supply of inert liquidmedium. Additional cooling may for example be achieved by using acatalytic zone in the form of a multitubular reactor with a fixed bed ofoxidation catalyst particles inside the tubes or on the shell side ofthe tubes and supplying coolant to the other side of the tubes. In aslurry bubble column, additional cooling may be achieved by providingthe bubble column with cooling coils.

The feed gas stream comprising H₂S and the inert liquid medium willtypically be supplied separately to the reaction zone. Alternatively,the feed gas stream comprising H₂S may be contacted with the inertliquid medium before they are supplied together to the reaction zone. Inthat case, part or all of the H₂S may be dissolved in the inert liquidmedium that is supplied to the reaction zone.

Each catalytic zone in the reaction zone of the process according to theinvention may be in any form that is suitable for a three-phase reactionsystem, for example a trickle flow fixed catalyst bed or a slurry bubblecolumn.

Reference herein to a H₂S-depleted gas stream is to a gas stream whereinthe H₂S concentration is lower than the H₂S concentration in the feedgas stream comprising H₂S. It will be understood that the H₂Sconcentration in the H₂S-depleted gas will depend on the H₂Sconcentration in the feed gas stream and on the conversion of H₂S.Preferably, the H₂S conversion in step (a) is at least 40%, typically inthe range of from 40 to 99.99%. Typical ranges of H₂S concentration inthe H₂S depleted stream are from 0.001 ppmv to 1 vol %, based on thetotal gas stream. Preferably, the H₂S concentration in the H₂S depletedstream is less than 1 ppmv.

The overall molar ratio of oxygen in the molecular-oxygen comprising gasand H₂S in the feed gas that are supplied to the reaction zone ispreferably in the range of from 0.3 to 3.0, more preferably of from 0.5to 2.0. In order to achieve deep desulphurisation, i.e. to obtain a H₂Sdepleted gas having less than 1 ppmv of H₂S, the overall molar ratio issuitably at least slightly above the stoichiometric ratio of 0.5. Thus,an oxygen—to—H₂S ratio in the range of from 0.6 to 1.5 is particularlypreferred.

The oxygen concentration in the molecular-oxygen comprising gas is notcritical. It will be appreciated that the preferred oxygen concentrationdepends primarily on the concentration of the H₂S in the H₂S comprisinggas. In the case of a very high content of H₂S in the feed gas it ispreferred, in order to avoid a high concentration of nitrogen or othergases in the H₂S depleted gas, to either use substantially pure oxygenor to use air in an embodiment of the process wherein reactions (3a) and(3b) are performed in separate catalytic zones. Examples of suitablemolecular-oxygen comprising gases are oxygen, air or oxygen-enrichedair.

In the process according to the invention, the temperature in eachcatalytic zone is in the range of from 120 to 160° C., preferably offrom 125 to 150° C.

The present invention can be used to selectively oxidize H₂S in step (a)from various gaseous streams, for example light hydrocarbons, such asmethane, ethane, propane, and gases derived from such lighthydrocarbons; natural gas; gases derived from tar sand and shale oils;gases associated with crude oil production; coal derived synthesis gas;gases such as hydrogen, nitrogen, carbon monoxide, carbon dioxide andmixtures thereof; steam; inert gases such as helium and argon; andproduct gas streams from other H₂S removal processes that containresidual H₂S.

The reaction zone in step (a) of the process according to the inventionis preferably operated at elevated pressure, more preferably a pressurein the range of from 2 to 200 bara, even more preferably in the range offrom 10 to 150 bara. Most preferably, the operating pressure is in therange of from 60 to 120 bara. In those embodiments wherein reactions(3a) and (3b) are performed in separate catalytic zones, it might beadvantageous to operate catalytic zone B (reaction (3b)) at a lowerpressure than catalytic zone A (reaction (3a)).

It is an advantage of the process of the invention that the feed gasstream comprising H₂S can be oxidised at the pressure at which it isproduced or at which it becomes available. If the feed gas stream is forexample natural gas, it can be processed in step (a) at the pressure atwhich it is produced at the well and effluents from a hydroprocessing orgasification unit can be processed without depressurizing them.

In all embodiments, the sulphur formed in step (a) of the processaccording to the invention is essentially in liquid form. Essentially inliquid form means that the degree of sulphur polymerization is limitedto the extent that the sulphur can still be removed from the reactionzone with the inert liquid medium, such that there is no build-up ofsulphur on the catalyst to the extent that sulphur prohibits access ofthe reactants to the catalytically active sites. Therefore, thetemperature in the at least one catalytic zone is at most 160° C.

The elemental sulphur formed in step (a) is removed from the reactionzone with inert liquid medium to obtain a liquid stream comprising inertliquid medium and essentially liquid elemental sulphur.

In all embodiments of the invention, the inert liquid medium is suppliedto each catalytic zone and thus in each catalytic zone, the reactionsaccording to equations (3a) and/or (3b) always take place in thepresence of inert liquid medium. The inert liquid medium may be anyliquid medium that is not substantially consumed under the processconditions and that does not substantially degrade the oxidationcatalyst. At least part of the inert liquid medium should be in liquidform at the process conditions in order to be able to control theprocess temperature and to remove the sulphur formed from the reactionzone. The inert liquid medium may be a reaction product of the selectiveoxidation reaction (3), i.e. water or liquid sulphur. The inert liquidmedium may also be another liquid compound that is not substantiallyconsumed under the process conditions. Examples of such liquids areparaffins like n-pentane, n-hexane, n-heptane, n-octane and mixturesthereof, refinery hydrocarbon streams such as naphtha or kerosine, crudeoil, toluene, alkanol amines and sulfinol. The inert liquid medium ispreferably elemental sulphur. Liquid sulphur is a particular suitableinert liquid medium, because it avoids the need for separation ofsulphur from the inert liquid medium and the inevitable separationlosses. Thus, in the event that liquid sulphur is used as inert liquidmedium step (b) is omitted.

In the event that the inert liquid medium is not liquid sulphur, it ispreferred to separate the liquid stream obtained in step (a) into afirst liquid phase enriched in inert liquid medium and a second liquidphase enriched in liquid elemental sulphur according to step (b). Theseparation may be achieved using any suitable separation technique, forexample using a solid/liquid separator or using a density separator.Preferably, the separation is done without decompressing the liquidstream obtained in step (a). Preferably, the second liquid phaseobtained in step (b) comprises elemental sulphur in the range of from 40wt % to 100 wt %, more preferably of from 50 wt % to 100 wt %, based onthe total weight of the second liquid phase.

In step (c) of the process according to the invention, at least part ofthe liquid stream obtained in step (a) or at least part of the secondliquid phase obtained in step (b) is subjected to a combustion step,thereby forming a fluid stream comprising sulphur dioxide. At least partof the liquid stream obtained in step (a) or at least part of the secondliquid phase obtained in step (b) is introduced into a combustion zone,typically by using compressed air. In the combustion zone, sulphur isburned to SO₂ according to reaction (4):

S+O₂→SO₂+heat  (4)

An unwanted side reaction is the formation of SO₃ according to reaction(5):

SO₂+½O₂SO₃+heat  (5)

When SO₃ reacts with water it will form sulphuric acid which can resultin downstream corrosion problems. To avoid or suppress the undesiredformation of SO₃, the SO₂ concentration and the combustion temperaturein the combustion zone are kept at a high level. Preferably, the SO₂concentration in the combustion zone is at least 5 vol %, morepreferably at least 15 vol %. When using air in the combustion process,the maximum theoretical SO₂ concentration in the combustion chamber is21 vol %. It has been found that an SO₂ concentration of 18 vol % givesan SO₃ concentration as low as 0.1%. Typically, the combustiontemperature is in the range of from 800 to 1500° C., preferably from1000 to 1300° C. This ensures that the formation of SO₃ is suppressed.

An advantage of the process according to the invention is that theamount of elemental sulphur, which would normally be discarded as waste,is considerably reduced.

Another advantage is that the heat released in the combustion processcan be recovered. Typically, heat recovery is effected by passing thesecond gas stream comprising hot SO₂ gas from the combustion zonethrough a waste heat boiler, generating steam. The hot steam can then beused for example to generate power that can be used elsewhere in theprocess or outside the process.

Yet another, even more important advantage is that the process accordingto the invention enables the removal of mercaptans in the event that thefeed gas stream comprises mercaptans, to produce a gas stream depletedof H₂S as well as of mercaptans. Generally, mercaptans are moredifficult to remove from the gas stream than H₂S, especially when it isdesired to remove mercaptans to low levels. This problem is solved bythe process according to the invention, as it allows the removal ofmercaptans to low levels, in the ppm range and for some mercaptans evenin the ppb range. Mercaptans will not be selectively oxidised in step(a), but they will be removed from the reaction zone together with theliquid stream comprising inert liquid medium and essentially liquidelemental sulphur obtained in step (a). Without wishing to be bound byany specific theory on mercaptan removal, it is believed that mercaptansare absorbed into the essentially liquid sulphur obtained in step (a)and/or are converted to polysulphides. In step (c), mercaptans and/orpolysulphides will then be combusted to form carbon dioxide, water andSO_(x). Thus, the process according to the invention enables theproduction of a gas stream depleted of H₂S and depleted of mercaptans,because the mercaptans will to a large extent be dissolved in the liquidelemental sulphur and will thus be removed from the feed gas stream,resulting in a gas stream obtained in step (a) depleted of H₂S as wellas of mercaptans.

Reference herein to mercaptans is to aliphatic mercaptans, especiallyC₁-C₆ mercaptans, more especially C₁-C₄ mercaptans, aromatic mercaptans,especially phenyl mercaptan, or mixtures of aliphatic and aromaticmercaptans. The invention especially relates to the removal of methylmercaptan, ethyl mercaptan, normal- and iso-propyl mercaptan and butylmercaptan isomers.

The process according to the invention is suitable for feed gas streamscomprising H₂S and mercaptans, wherein the concentration of mercaptansis in the range of from 1 ppmv to 1 vol %, based on the total feed gasstream. It will be understood that the mercaptan concentration in theresulting H₂S-depleted gas stream gas stream obtained after step (a)will depend on the mercaptan concentration in the feed gas stream.Typical mercaptan concentrations in the H₂S-depleted gas stream gasstream obtained after step (a) will be in the range of from 100 ppbv to0.1 vol %.

Preferably, the process comprises additional step (d) of injecting thefluid stream comprising sulphur dioxide obtained in step (c) into anacid gas field. This offers the advantage that no SO₂ is emitted to theatmosphere. The injection of SO₂ requires a high pressure. The pressureneeded depends on several factors, inter alia on the composition of theacid gas field and on the depth of injection aimed for. Generally, thepressure needed is in the range of from 5 bara to 300 bara (barabsolute). An advantage of the process according to the invention isthat, because step (a) is done at elevated pressures, the liquid streamcomprising inert liquid medium and essentially liquid elemental sulphurobtained in step (a) is pressurised. This will result in a pressurisedfluid stream comprising SO₂. No compression equipment or only smallcompression equipment will therefore be needed for the injection of SO₂fluid stream into the acid gas field. Typically, the pressure of theliquid stream comprising inert liquid medium and essentially liquidsulphur obtained in step (a) is in the range of the operating pressureof the oxidation zone in step (a), typically from 2 to 200 bara, evenmore preferably in the range of from 10 to 150 bara, most preferablyfrom 60 to 120 bara. The second gas stream comprising SO₂ will typicallyhave a pressure slightly below the pressure of the liquid streamcomprising inert liquid medium and essentially liquid sulphur obtainedin step (a), typically 1 to 5 bara lower.

Preferably, SO₂ is injected into the acid gas field in the form of asolution. Solvent is then added to the fluid stream comprising sulphurdioxide obtained in step (c) to form a fluid stream comprising sulphurdioxide and the solvent. This fluid stream is then injected into theacid gas field.

The solvent can be water or an organic solvent. Suitable organicsolvents are the lower alcohols such as methanol, ethanol and propanol,acetone or hydrocarbons.

SO₂, when injected into the acid gas field can react with H₂S present inthe acid gas field to form sulphur according to reaction (6):

2H₂S+SO₂→2H₂O+3/nS_(n)  (6)

If the H₂S in the acid gas field is relatively concentrated, typicallyin amounts greater than 3 to 5%, in the gas phase and water is employedas solvent, the reaction occurs at a relatively rapid rate. If anorganic solvent is used and conditions are otherwise the same, thereaction is generally somewhat slower. In this way, the reaction can besteered, for example to form a layer of sulphur in the acid gas field ata location a substantial distance away from the point of injection. Thislayer of sulphur can be used as a barrier or plug in the acid gas field,to seal off part of the acid gas field. Misdirected placement of asulphur layer around the point of injection may be avoided by injectinga solvent, for example alcohol, which is miscible in the acid gas field,optionally followed by injection of a suitable hydrocarbon. This willreduce water saturation near the point of injection and will furthermoredrive away H₂S from the point of injection, thereby preventing theformation of sulphur near the point of injection. It will be understoodthat the exact procedure to be used depends inter alia upon the intendeduse of the resulting sulphur layer, the characteristics of the acid gasfield and the size of the acid gas field.

EXAMPLES FOR STEP (A)

Step (a) of the invention will now be illustrated by the followingnon-limiting examples 2 to 4.

Example 1 (Comparative) Catalyst Preparation

Silica extrudates having a surface area of 358 m²/g as measured bynitrogen adsorption (according to the BET method) and a pore volume of1.34 ml/g as measured by mercury intrusion were provided with hydratediron oxide. 100 grams of the silica extrudates were impregnated with 134ml of a solution prepared from 28.6 grams of ammonium iron citrate(containing 17.5 wt % iron) and de-ionized water. The impregnatedmaterial was rotated for 90 minutes to allow equilibration. The materialwas subsequently dried at 60° C. for 2 hours, followed by drying at 120°C. for 2 hours and calcinations in air at 500° C. for 1 hour. Theinitial colour of the catalyst was black, but turned into rusty browndue to hydration of iron oxide. The resulting catalyst had a surfacearea of 328 m²/g, a pore volume of 1.1 ml/g and an iron content of 4.7wt % based on the total catalyst weight.

Selective Oxidation

3 grams of the catalyst prepared as described above were diluted with0.1 mm silicon carbide particles to achieve a volume ratio of siliconcarbide/catalyst particles of 1.67. This mixture was loaded into areactor tube with an internal diameter of 10 mm, fitted with a 4 mminternal thermowell. The loaded reactor tube was mounted into a reactionsystem comprising an oven to preheat the feed and control the catalysttemperature. The reaction system furthermore comprised mass flowcontrollers (MFC) for dosing the various gases, a liquid supply system,a high-pressure gas-liquid separation vessel, a liquid level controllerin combination with a valve to release the liquid effluent, a constantgas pressure valve and a wet gas meter.

At the start of the experiment, the reactor was pressurized with a flowof nitrogen to the reaction pressure of 30 bar g and the temperature wasset at 135° C. The nitrogen flow was stopped and a feed gas comprising15 vol % H₂S in methane and a gas comprising 4 vol % of molecular oxygenin helium were supplied to the reactor at flow rates of 3.1 and 5.9Nl/h, respectively. Within 24 hours after start of the feed gas supply,the reactor was plugged as was evident from the absence of any gas flow.Unloading the reactor at room temperature revealed that solidifiedsulphur was formed, which had caused clogging of the catalyst.

Example 2 (According to the Invention)

A reactor tube was loaded with catalyst and mounted in a reactor systemas described in Example 1. The reactor was pressurized to a pressure of30 bar g using a nitrogen flow. Toluene was then supplied to the reactortube continuously at a rate of 30 grams/hour and the temperature of thetube was set at 135° C. The nitrogen flow was stopped and a feed gascomprising 15 vol % H₂S in methane and a gas comprising 4 vol % ofmolecular oxygen in helium were mixed with the toluene stream to besupplied to the reactor tube at flow rates of 3.1 and 5.9 Nl/h,respectively, upstream of the oven.

After 48 hours at 30 bar g, the pressure was decreased to 15 bar g.

After 72 hours at 15 bar g, the pressure was increased to 90 bar g and afeed gas comprising 7 vol % H₂S in methane and a gas comprising 4 vol %of molecular oxygen in helium were mixed with the toluene stream to besupplied to the reactor tube at flow rates of 4.8 and 4.2 Nl/h,respectively.

After 48 hours on stream under these process conditions, pressure wasdecreased to 30 bar g and a feed gas comprising 15 vol % H₂S in hydrogenand a gas comprising 4 vol % of molecular oxygen in helium were mixedwith the toluene stream to be supplied to the reactor tube at flow ratesof 3.1 and 5.9 Nl/h, respectively. These conditions were maintainedduring 72 hours.

During the whole experiment, gaseous and liquid effluent werecontinuously removed from the reactor tube.

Samples of the gaseous effluent were taken before each change inpressure or feed gas composition and at the end of he experiment. Thesamples were analyzed using online gas chromatography and X-rayfluorescence (XRF). The H₂S and the methane conversion were calculated.The results are shown in Table 1.

The experiment clearly demonstrates that high H₂S conversions areachieved with the H₂S comprising methane feed gases at a temperature aslow as 135° C. and that the catalyst does not deactivate over time.Furthermore it is demonstrated that oxygen reacts very selectively withthe H₂S in that the conversion of CH₄ is very low.

Example 3 (According to the Invention) Catalyst Preparation

A precipitated iron oxide on silica powder, denoted as ABS 50 with anominal composition of 50% wt Fe₂O₃ and 50% wt SiO₂, a particle sizeD[v,50] of 10 micron and a BET surface area of 270 m²/g, was obtainedfrom Euro Support B.V. (Amersfoort, NL). The powder was treated in airat 450° C. for 2 hours, cooled down to room temperature. The resultingpowder is used as catalyst A.

Selective Oxidation

A 250 ml autoclave reactor equipped with a magnetically coupled stirrer,a gas manifold to supply metered amounts of a gas via two separate diptubes, a back-pressure regulator, a wet gas test meter and an on-linegas chromatograph was used for the selective oxidation experiment. Theautoclave was filled with 306 grams of solid sulphur and 20.3 grams ofcatalyst A. The autoclave was heated to 135° C. After 2 hours, thestirrer was started at 800 rpm. The vessel was pressurized to 40 bar gusing a gaseous stream of 7 vol % H₂S in methane which was fed via thedip tube below the liquid level. When the pressure level was reached,the feed gas flow (7 vol % H₂S in methane) was adjusted to 4.2 Nl/h anda gaseous stream of 4 vol % O₂ in helium was added via a separate diptube, also below the level of the liquid, at a rate of 6.0 Nl/h. TheO₂/H₂S ratio of the gases supplied to the autoclave was calculated as0.82 mole/mole and the gas hourly velocity as 510 Nl/kg catalyst/h.

After 30 hours, the feed gas flow is increased to 6.0 Nl/h, whichcorresponds to an O₂/H₂S ratio of 0.57.

After another 20 hours, the feed gas flow was decreased to 3.5 Nl/hcorresponding to a gas hourly velocity of 475 Nl/kg/h and a O₂/H₂S ratioof 0.98. After 72 hours, the experiment was stopped.

Samples of the gaseous effluent were taken before each change in feedgas flow and at the end of he experiment. The samples were analyzedusing online gas chromatography (equipped with a pulsed dischargedetector). The H₂S and the methane conversion were calculated. Theresults are shown in Table 1. The CO₂ concentration in the effluentsamples was less than 50 ppmv, indicating that oxidation of methane isvirtually zero.

Example 4 (According to the Invention) Catalyst Preparation

273.6 grams of the ABS 50 powder (see EXAMPLE 3) was mixed with 64.1grams of de-ionized water, 60 grams of a 5% wt aqueous solution of polyvinyl alcohol and 16 grams of ammonia (25%) to an extrudable dough witha solids content of 53.2 wt % and a pH of 9.5. This mix was extrudedusing a 1.6 mm diameter trilobe die-plate. The extrudates were dried at120° C. and calcined at 550° C. for 2 hours and used as catalyst B.

Selective Oxidation

A reactor system was used for the selective oxidation experiment, thesystem comprising:

a reactor tube;

a gas manifold to supply metered amounts of gases via two separate feedlines to the reactor tube;

a gas-liquid separator directly downstream of the reactor tube

a liquid recycle pump for recycling liquid from the gas-liquid separatorto the reactor tube;

a liquid holding vessel that is connected to the liquid recycle system,from which liquid can be supplied to the reactor tube and to whichliquid from the gas-liquid separator can be supplied; and

a back-pressure controller in the vapour effluent line from thegas-liquid separator. The entire reactor system was mounted into an ovenfor temperature control.

2.0 grams of catalyst B were diluted with an equal volume of SiC andloaded into the reactor tube, which was subsequently mounted into thereactor system. Solid sulphur (70 grams) was added to the liquid holdingvessel. The temperature of the reactor system was set at 135° C. Aftermelting, the liquid sulphur was added to the bottom part of thegas-liquid separator and the reactor system was pressurized with astream comprising 4 vol % O₂ in helium to 60 bar g. Then, the liquidsulphur was recycled over the catalyst bed and the sulphur flow wasmonitored by differential pressure measurement using a capillarycalibrated with oil at ambient pressure before the experiment. Feed gas(7 vol % H₂S in methane) and a gas comprising 4 vol % O₂ in helium weresupplied to the reactor tube at flow rates of 1.60 Nl/h and 1.63 Nl/h,respectively. This corresponds to a total gas hourly velocity of 1610Nl/kg/h and an oxygen/H₂S ratio of 0.56.

After 20 hours, the pressure was increased to 90 bar g and the gas andliquid flows were adjusted to increase the total gas hourly velocity to2250 Nl/kg/h while maintaining the same oxygen/H₂S ratio and increasethe ratio of gas/liquid flow rates.

After another 20 hours, the gas and liquid flows were adjusted tooperate at an oxygen/H₂S ratio of 1.22 and a total gas hourly velocityof 1590 Nl/kg/h.

Samples of the gaseous effluent were taken before each change inpressure or flow rates and at the end of the experiment. The sampleswere analyzed using online gas chromatography (equipped with a pulseddischarge detector). The H₂S conversion was calculated. The results areshown in Table 1.

TABLE 1 Process conditions and results of examples 2 to 4. gas/ liquidH₂S CH₄ O₂/H₂S inert liquid flow p conversion conversion example feedgas ratio medium (Nl/kg) (bar g) (%) (%) 2 a 15 vol % H₂S in CH₄ 0.5toluene 300 30 98.2 0.02 b 15 vol % H₂S in CH₄ 0.5 (continuous 300 15 88<0.02 c 7 vol % H₂S in CH₄ 0.5 supply 300 90 99.8 0.01 d 15 vol % H₂S inH₂ 0.5 without 300 30 71 n.a. recycle) 3 a 7 vol % H₂S in CH₄ 0.82sulphur n.a. 40 >99.9 <0.01 b 7 vol % H₂S in CH₄ 0.57 (batch) n.a. 40 85<0.01 c 7 vol % H₂S in CH₄ 0.98 n.a. 40 98.2 <0.01 4 a 7 vol % H₂S inCH₄ 0.56 sulphur 100 60 21 b 7 vol % H₂S in CH₄ 0.55 (continuous 200 9060 c 7 vol % H₂S in CH₄ 1.22 supply with 100 90 99.7 recycle) n.a.: notapplicable

Example 5 Mercaptan Removal (Comparative Example)

A 250 ml autoclave reactor equipped with a magnetically coupled stirrer,a gas manifold to supply metered amounts of a gas via two separate diptubes, a back-pressure regulator, a wet gas test meter and an on-linegas chromatograph was used for the mercaptan capture oxidationexperiment. No catalyst was added. The autoclave was heated to 135° C.After 2 hours, the stirrer was started at 800 rpm. The vessel waspressurized to 30 or 40 barg using a gas stream of methane which was fedvia the dip tube below the liquid level. When the pressure level wasreached, the feed gas flow was switched and adjusted to the desired flowrate (see table 2).

Samples of the gaseous effluent were taken before each change in feedgas flow and at the end of he experiment. The samples were analyzedusing online gas chromatography (equipped with a pulsed dischargedetector). In this experiment, all analysis indicated the presence ofH₂S in the effluent gas. The mercaptan conversion was calculated. Theresults are shown in Table 2.

Example 6 Mercaptan Removal in the Presence of H₂S (According to theInvention)

A precipitated iron oxide on silica powder with a nominal composition of50% wt Fe₂O₃ and 50% wt SiO₂, a particle size D[v,50] of 10 micron and aBET surface area of 270 m²/g, was obtained from Euro Support B.V.(Amersfoort, NL). The powder was treated in air at 450° C. for 2 hours,cooled down to room temperature.

A 2 cm cross section bubble column, height 25 cm reactor is mounted intoan oven and equipped with a glass frid to support the catalyst, a gasmanifold to supply metered amounts of a gas via two separate tubes belowthe glass frid, a back-pressure regulator, a wet gas test meter and anon-line gas chromatograph was used for the selective oxidationexperiment. The column was filled with 60 grams of solid sulphur and 3.0grams of catalyst. The bubble column was pressurized with nitrogen up to17 barg and heated to 139° C. When the pressure level and temperaturewas reached, the feed gas flow was switched to the mixture indicated inTable 2 at a total flow rate of 6 Nl/hr corresponding to a gas hourlyspace velocity of 2000 Nl/kg catalyst/hour. Samples of the gaseouseffluent (the H₂S depleted gas stream) were taken during the experiment.The samples were analyzed using online gas chromatography (equipped witha pulsed discharge detector). The H₂S and the mercaptan conversion werecalculated. The result is shown in Table 2. The results show that in theevent that the feed gas stream comprises mercaptans, these are removedmore effectively in the presence of a catalyst and molecular oxygen,resulting in a H₂S depleted gas stream having lower levels ofmercaptans.

TABLE 2 Process conditions and results of examples 5 and 6. gas H₂Smercaptan O₂/H₂S flow p conversion conversion example feed gas ratio(Nl/hr) (barg) (%) (%) 6 0.5 vol % H₂S, 0.0321 vol % 2.46 6 17 99.9 >99for C4H9SH CH3SH, 0.0336 vol % C4H9SH, >99 for CH3SH 1.23 vol % O2, 25vol % N2, 33.3 vol % CH4, 4.8 vol % nC5H12 and balance He 5 a 0.06 vol %CH₃SH in CH₄ 9.9 40 83 for CH₃SH b 0.046 vol % CH₃SH in CH₄ 5.2 40 88for CH₃SH c 0.014 vol % CH₃SH + 0.014 16.8 30 48 for CH₃SH; vol % C₄H₉SHin CH₄ >97 for C₄H₉SH

1. A process for producing a gas stream depleted of H₂S from a feed gasstream comprising H₂S, the process comprising the steps of: (a)selectively oxidizing H₂S by supplying the feed gas stream comprisingH₂S, an inert liquid medium and a molecular-oxygen comprising gas streamto a reaction zone comprising at least one catalytic zone comprising anoxidation catalyst and contacting the oxidation catalyst of each of theat least one catalytic zone with the feed gas stream and themolecular-oxygen comprising gas stream in the presence of the inertliquid medium at a temperature in the range of from 120 to 160° C. toform elemental sulphur and the gas stream depleted of H₂S, under suchconditions that the elemental sulphur formed is essentially in liquidform and is removed from the reaction zone along with the inert liquidmedium to obtain a liquid stream comprising the inert liquid medium andessentially liquid elemental sulphur; (b) separating the liquid streamobtained in step (a) into a first liquid phase enriched in the inertliquid medium and a second liquid phase enriched in liquid elementalsulphur; (c) combusting either at least part of the liquid streamobtained in step (a) or at least part of the second liquid phaseobtained in step (b); or both; to form a fluid stream comprising sulphurdioxide.
 2. A process according to claim 1, further comprising the stepof (d) injecting the fluid stream comprising sulphur dioxide obtained instep (b) into an acid gas field.
 3. A process according to claim 1,wherein a solvent is added to the fluid stream comprising sulphurdioxide to form a fluid solvent stream comprising sulphur dioxide thatis injected into the acid gas field.
 4. A process according to claim 1,wherein the second liquid phase obtained in step (b) comprises elementalsulphur in the range of from 40 wt % to 100 wt %, based on the totalweight of the second liquid phase.
 5. A process according to claim 1,wherein the concentration of H₂S in the feed gas stream is in the rangeof from 0.5 to 50 vol %.
 6. A process according to claim 1, wherein theinert liquid medium is elemental sulphur.
 7. A process according toclaim 1, wherein the operating pressure in each of the at least onecatalytic zone in step (a) is in the range of from 2 to 200 bara.
 8. Aprocess according to claim 1, wherein the feed gas stream furthercomprises mercaptans and at least part of the mercaptans is removed fromthe feed gas stream to produce a gas stream depleted of H₂S and ofmercaptans.
 9. A process according to claim 8, wherein the concentrationof mercaptans in the feed gas stream is in the range of from 1 ppmv to 1vol %, based on the total feed gas stream.